This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. We are using a new mass spectrometric method for determining protein stoichiometries in complexes. The technique involves the production of heavy atom labeled artificial proteins composed of "tryptic fragment representatives" of each of the proteins in the complex of interest. To date several of the needed artificial proteins have been produced and characterized.